As disclosed in JP-A-2003-34233, an anti-theft vehicle alarm system has been developed that produces an alarm if an attempt is made to enter a vehicle, break a window of the vehicle, or jack-up the vehicle. For example, the alarm system includes a tilt sensor to detect a tilt angle of the vehicle in order to determine whether the vehicle is jacked-up. The tilt sensor is an acceleration sensor that has a detection axis parallel to ground to detect the acceleration of gravity in order to the tilt angle of the vehicle.
Typically, the alarm system switches to an alarm mode after a driver parks the vehicle and locks a door of the vehicle. Then, when the driver returns to the vehicle and unlocks the door, the alarm system leaves the alarm mode. The alarm system includes a microcomputer that stores a tilt sensor output as a reference value at the moment in time the door is locked. The microcomputer compares the reference value with each output of the tilt sensor while the vehicle is parked. Thus, the microcomputer calculates a change in the tilt angle of the vehicle. If the change exceeds a predetermined range, the alarm system produces the alarm.
The microcomputer is powered by a first power source and the tilt sensor is powered by a second power source separated from the first power source. The first power source generates a first voltage from a battery voltage and supplies the first voltage to the microcomputer. The second power source generates a second voltage from the battery voltage and supplies the second voltage to the tilt sensor. In this case, the second power source supplies the second voltage to the tilt sensor intermittently in order to cause the tilt sensor to operate intermittently. This approach reduces the amount of current consumed during the periods the vehicle is parked.
When the alarm system switches to the alarm mode, the microcomputer stores the tilt sensor output as the reference value, stops the second power source, and then switches to a sleep mode. Then, after a predetermined time period (e.g., 100 microseconds) has been elapsed, the microcomputer wakes up from the sleep mode. Then, the microcomputer starts the second power source, receives the tilt sensor output, and calculates the change in the tilt angle of the vehicle. If the change is within the predetermined range, the microcomputer stops the second power source and switches to the sleep mode again. The microcomputer repeats this intermittent operation until the change is outside the predetermined range. If the change is outside the predetermined range, the alarm system produces the alarm.
Typically, in the alarm system, as the battery voltage decreases, the first and second voltages decrease accordingly. In this case, the second voltage supplied to the tilt sensor begins to decrease earlier than the first voltage supplied to the microcomputer.
The microcomputer converts the tilt sensor output to digital value by using a ratio between the tilt sensor output and the first voltage. When the second voltage changes, the tilt sensor output changes accordingly. Therefore, the tilt sensor output may deviate from its true value. As a result, a false alarm may be activated.
To overcome the above problem, an apparatus disclosed in JP-2889992 includes detection means for detecting the battery voltage. When the battery voltage decreases below a threshold value during the alarm mode, the alarm process is halted and the tilt sensor is initialized. The initialization clears the stored tilt angle in order to prevent the false alarm to be activated. Then, the alarm process restarts.
However, the apparatus has problems to be overcome. The decreases in the first and second voltages depend on designs of the first and second power sources and environmental factors such as temperature. Therefore, the threshold value needs to be set to a high value.
Further, in the apparatus, the second power source for supplying the second voltage to the tilt sensor is separated from the first power source for supplying the first voltage to the microcomputer. Therefore, when the second voltage decreases due to a breakdown in the second power source, the detection means cannot detect the decrease in the second voltage. As a result, the false alarm may be activated.